Electron gun for electron discharge tube



March 7, 1961 Filed March 18, 1954 A. H- W. BECK ET AL ELECTRON GUN FOR ELECTRON DISCHARGE TUBE 2 Sheets-Sheet 1 Inventor:

A.H.W. BECK- DCROGERS' P. F-C.BURKE ygdgk y 7/1 64% A ttorney March 7, 1961 A. H. w. BECK ET AL 2,974,246

ELECTRON GUN FOR ELECTRON DISCHARGE TUBE Filed March 18, 1954 2 Sheets-Sheet 2 F/ G Z I 3 2 47 7: 45 5/ 46 3 54 Ziqg A 42 x 43 I g 7,540 I l 4' 5a 48 1 49 49 64 A lse L Inventors A.H.W.BECK-D.C.ROGERS- P.F.c. BURKE A tforney screw thread working in a fixed nut.

United States Patent 2,974,246 ELECTRON GUN FO%II.II;IICTRON DISCHARGE Arnold Hugh William Beck, Douglas Cecil Rogers, and

Peter Francis Conway Burke, London, England, assignors to International Standard Electric Corporation, New York, N .Y.

Filed Mar. 18, 1954, Ser. No. 417,116

Claims priority, application Great Britain Mar. 26, 1953 Claims. (Cl. 313-84) The present invention relates to electron guns for projecting electron beams of high current density along rectilinear paths and is particularly concerned with modifications and improvements in electron guns forming the subject matter of British Patent No. 674,758 granted October 22, 1952.

. The specification of the above mentioned Patent No. 674,758 describes and claims an electron beam focussing arrangement comprising an electron gun structure comprising a centrally apertured magnet pole piece, a cathode and a focussing electrode within the said pole piece and an apertured anode of non-magnetic material secured in the aperture of the said p'ole piece.

The parent invention is particularly useful in connection with travelling wave tubes in which an electron beam is projected along the axis of a retardation waveguide, such as a helix, to interact with electromagnetic waves therein. The parent specification illustrated such use, a beam focussing magnetic field being set up between a pair of pole pieces, one at either end of the helix, one pole piece being used as a housing and magnetic screening means for the electrostatic focussing part of the arrangement, thus providing an electron gun of which the anode was the front end plate of the pole pieces. In the embodiment there illustrated, this pole piece-anode also provided part of the envelope of the electron discharge tube.

In travelling wave tubes developed for commercial use, we have, up to now, preferred to use a glass envelope surrounding the electron gun and, therefore, when it is desired to utilize the parent invention, for such tubes we prefer to modify the gun construction accordingly. What is more important, however, is that the parent invention provides a practical means of launching a particular type of electron motion known as Brillouin flow, and, in the present invention the electron gun arrangements of the parent specification are modified as a result of our experience in realising Brillouin flow in travelling wave tubes.

In the parent specification reference is made to a form of magnetic focussing in which there is a fall of potential towards the centre of the electron beams-4n other words, electrons near the centre of the beam have a lower axial velocity than those near the beam boundary. The electrons travel in helices, each about a magnetic line of force, so that the electrons are constrained to paths tending to coincide with the magnetic lines of force. In Brillouin flow, on the other hand, the electrons have helical paths about the beam axis instead of about individual lines of force, the Lorenz forces on the electrons counterbalancing the centrifugal force on the electrons and also the mutual repulsion between them. In a solid, as compared to an annular, beam of electrons in Brillouin flow,

ties, the beam moving like a solid rotating rod having a The magnetic field required for Brillouin flow is, in general, less than required for the first above mentioned type of flow. .Of

the electrons all have the same angular and axial veloci- Patented Mar. 7, 1961 parallel, or very nearly parallel, beam of electrons abruptly from a magnetic field-free region into the axial magnetic field required to sustain the desired flow. The beam, therefore, is projected through an aperture in a pole piece of the field producing magnet arrangement. This, of course, is exactly what happens in the focussing arrangement of the parent invention when the strength of the magnetic field is suitably adjusted for the Brillouin flow instead of for what may be aptly called the magnetic constraint type of flow.

In the immediate neighbourhood of the pole piece aperture the magnetic lines of force are curved instead of being wholly axial, while some of the field penetrates a short distance into the aperture. This curvature, together with the slight non-parallelism of the electrons on either side of the electrostatic throat formed at the beam exit from the tunnel in the pole piece-anode of the parent invention, providing the necessary orthogonal components of magnetic field and electron velocity for imparting to the electrons their necessary rotation about the beam axis.

It will be appreciated that at the transition region in which the electron beam enters the magnetic field the gion. The theory of this matter is not yet fully understood, nevertheless it is found, in practice, that better focussing results are obtained if the magnetic pole piece aperture is, in fact, larger than the aperture in the electrostatic anode.

According to one aspect of the present invention therefore, there is provided in or for an electron beam focussing arrangement according to Patent No. 674,758 an electron gun structure comprising a centrally apertured magnet pole piece, a cathode and a focussing electrode within the said pole piece and an apertured anode of non-magnetic material secured in the aperture of the said pole piece.

In connection with the transition region discussed above, we have also determined that the throat at which the beam is first parallel to the axis should be formed slightly in front of the plane of emergence of the beam from the pole piece.

In accordance with a further aspect of the present invention, therefore, there is provided an electron beam focussing arrangement according to Patent No. 674,758 designed to converge the electrons through an exit aperture in a magnet pole piece so as to move parallel to the beam axis at a plane spaced from the said exit aperture in the direction of beam propagation a distance between one tenth of and twice the radius of the said exit aperture.

According to another aspect, the present invention provides an electron gun comprising: a cathode, an apertured anode of non-magnetic material and a focussing electrode surrounding the cathode arranged together to focus a space-charge-limited electron beam from the cathode through the anode aperture to an electrostatic throat slightly beyond the anode; and magnetic shielding means comprising an end plate apertured to receive the said anode.

As mentioned earlier, in embodiments of the present invention for use with travelling wave tubes it is at present preferred to contain the complete gun within the glass envelope of the tube.

One function of the magnetic pole piece within which the electron gun of the present invention is situated is to provide magnetic shielding means about the electrostatic beam forming electrodes. In practice the magnetic field penetrating to the cathode cannot be reduced to zero; some small percentage will remain however perfect the screening. It is known that, for the same total thickness of ferromagnetic shielding material, greater attenuation is obtained by providing, in succession, a plurality of shields separated by non-magnetic media than were a single thick shield to be used. In certain embodiments of the invention, therefore, inner and outer spaced-apart shielding means is provided about the cathode.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:

Fig. 1 shows, partially sectionalised, travelling wave apparatus including a travelling wave tube incorporating an electron gun according to the present invention.

Fig. 2 shows in section details of an electron gun according to the invention and Fig. 3 shows a modified construction of the electron gun of Fig. 2.

In Fig. 1 reference 1 indicates a travelling wave tube having an electron gun 2 housed within an envelope bulb 3, a helix 4 supported within a tubular extension 5 of the envelope and a collector electrode 6 which closes the end of the envelope portion 5 remote from the electron gun. The electron gun is shown only in outline and will be more fully described later. The collector electrode 6 fits into a cooler assembly carrying heat dissipating fins 8 and insulatingly supported at 9 in a magnet assembly end plate 10 which forms an armature and pole piece at this end of the tube. Envelope portion 3 carries at its rear end a base 12 fitted with conventional valve pins 13. The base 12 is secured to a plate 14 fixed by means of spacing collars 15 to an end plate 16 of a magnet assembly 17, which is clamped between the plate 16 and the aforementioned end plate 10 by means of tie rods 18. The magnet assembly further comprises a set of solenoids 19, 20, 21 and 22 solenoids and 21 being spaced apart by means of a brass collar 23, while an input wave guide 24 is secured between solenoids 19 and 20. Wave guide 24 has an extension 25 provided with a short-circuiting piston 26 and piston actuating screw 27, for the purpose of adjusting the impedance match between the wave guide 24 and a helix 4. The travelling wave tube 1 projects through the enclosure of wave guide 24 and its extension 25 so that a choke member 28 internal to the tube envelope is in line with an outer choke member 29 which is secured to the wave guide wall opposite to the end of the helix 4. Inner choke member 23 carries a short tube 30 secured to its front face and a tube 31 secured to its rear, the electron beam passing through the tubes 31 and 30. The end of the helix 4 is joined to the tube 30 which provides an antenna probe for coupling between the wave guide 24 and the helix 4. The helix 4 and inner choke member 28 are supported on quartz rods 32 within the envelope portion 5 of the travelling wave tube, while the tube 31 is secured to an insulating member 33 carried on the electron gun 2 so as to aiford independent D.C. connection means to the helix 4.

The arrangement at the output end of the helix are similar in general to those described, there being provided an output wave guide 34 and an impedance matching section 35 adjusted by means of a piston actuating screw 36. The helix matching arrangements are similar to those described for the input end of the helix, an

outer high frequency choke member 37 being visible in Fig. 1 adjacent collector electrode 6, to which, however, it is not connected.

Surrounding the electron gun end of the tube is a cylinder 38 of ferromagnetic material secured by means of a circular flange 39 to the end plate 16 of the magnet assembly. The outer member 40 of the electron gun 2 constitutes a pole-piece of the magnet system, separated by the air gap from the surrounding cylinder 38. The member 40 also serves as an additional magnetic shield surrounding the electron gun 2. To improve the magnetic field attenuation the cylinder 38 projects considerably to the rear of the plate 16, any external leakage flux tending to be collected on this rearward extension.

Fig. 2 shows a cross-section of the electron gun 2 of Fig. l. The gun comprises a cathode 41, shown in outline in the drawing, having a concave emitting surface 42. The cathode is surrounded by a focussing electrode 43 and cooperates with an anode 44 to form an electrostatic beam forming system as discussed earlier in the specification. The cathode 41 and focussing electrode 43 are surrounded by magnetic shielding means provided by the cylindrical pole piece member 40 of ferromagnetic material. The end of the cylindrical pole piece opposite the cathode is closed by an end plate 45, which in the embodiment of Fig. 2 is integral with the remainder of the member 40. The end plate 45 is centrally apertured to receive the anode 44 which is secured to it by means of screws 46. The aperture 47 in the anode 44 is determined from electrostatic considerations and since anode 44 is inset in the pole piece aperture, it is of smaller diameter than this aperture.

The cathode 41 and focussing electrode 43 are secured to an insulator 48 which seats against an internal shoulder 49 in the cylindrical wall of the pole piece 40. The pole piece 40 is located within the portion 3 of the discharge tube envelope by means of annular insulating washers 50, 51, and 52. Connecting leads 53 from the cathode, the focussing electrode and the pole piece member 40 are taken out through a glass press 54, as in conventional radio tube practice.

The upper insulating washer 52 carries a metallic support member 55 into which fits the tube 31 whose other end is secured to the inner choke member 28 of Fig. 1. so providing independent connection means for the helix. It is common practice in travelling wave tubes of the prior art to maintain the helix at the same potential as the final anode of the electron gun system. in preferred embodiments of the present invention, as exemplified in that of Fig. 2, provision is made for the helix to be maintained at a difierent DC. potential to that of anode 44. A connection 56 is thus made to the member 55 and taken out through a channel 57 in the magnetic shielding member 40 to the press 54.

In a practical embodiment of the electron gun shown in Fig. 2 (in which, it should be explained, the electrodes have not been drawn strictly to scale) the principal dimensions and characteristics of the electron gun were as follows:

Electron beam current ma, Beam diameter at throat "in 0.066 Efiective area of cathode surface 42 sq. in 0.062 Radius of curvature of surface 42 0.375 Anode-cathode voltage volts 2,000 Anode aperture (47, Fig. 2) in 0.18 Anode-cathode separtion in- 0.174 Pole piece aperture "in 0.313 Thickness of pole piece cylinder ..in 0.170 Thickness of end plate 45 in 0.150 Length of pole piece cylinder in l The magnetic design of the structure was arranged for use with a magnet system producing an axial magnetic field of about 520 oersteds.

In Fig. 3 there is shown an embodiment of the invention in which the magnetic screening of the cathode is provided by spaced-apart inner and outer shielding members, 58 and 59 respectively. The general arrangement of the gun is very similar to that of Fig. 2 but in so far as the various gun electrodes are concerned, the two drawings are not to the same scale nor in fact is either drawing strictly to scale. Components of Fig. 3 which in other respects are similar to those of Fig. 2 need not be further described and are identified by the same reference numerals.

The outer magnetic shielding member 59 is generally similar to the pole piece member 443 of Fig. 2 but may be made thinner. The anode 68, of copper, is a cylinder having a central circular flange 61. The inner shielding member 58 is generally cylindrical and is secured to the underside of the flange 61 of the anode 60 by means of screws 62 of non-magnetic material which clamp to gether the members 58 and 59 with the anode in between. The cathode 41 and focussing electrode 43 in this embodiment are mounted upon an insulated washer 63 which is seated against an internal circular shoulder 64 near the end of the inner shielding member 58.

While the invention has been described with particular reference to use in travelling-wave apparatus, the invention is not, of course, limited thereto, but may equally well be applied to electron guns for other types of discharge apparatus where it is desirable for the cathode to be shielded from a magnetic beam focussing field.

While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. An electron gun comprising: a cathode, an apertured anode of non-magnetic material, an electrostatic throat piece beyond said anode, and a focussing electrode surrounding the cathode arranged together to focus a space-charge-limited electron beam from the cathode, through the anode aperture to said electrostatic throat, and magnetic shielding means surrounding the anode, the cathode and focussing electrode to form a pole piece of a beam focussing arrangement which is external to the gun, the said magnetic shielding means comprising an end plate having an aperture larger than at least a first portion of said anode, and means for securing a second portion of said anode to said end plate.

2. An electron gun according to claim 1 in which the said shielding means comprises a hollow cylinder of ferromagnetic material partially closed at one end by the said end plate, and in which said first portion of said anode extends through said aperture in said end plate, said gun further comprising means for locating the said cathode and focusing electrode within the said cylinder, and means for mounting the gun in accurately spaced relation thereto within a surrounding glass envelope.

3. An electron gun according to claim 1 in which the said shielding means comprises inner and outer spacedapart ferromagnetic members surrotmding the cathode and focussing electrode.

4. An electron gun according to claim 3 in which the said outer ferromagnetic member is a hollow cylinder partially closed at one end by the said end plate, the said anode is a tubular member secured to the said end plate and extending into the aperture in said end plate 6 and a said inner ferromagnetic member is secured to the said anode.

5. An electron gun according to claim 2 in which the said cylinder has an internal shoulder for locating an insulator carrying the said cathode and focussing electrode and in which an insulator is secured over the said end plate for locating the gun within a surrounding envelope.

6. An electron gun according to claim 3 comprising insulating means secured to the said end plate supporting a connecting member for a discharge tube electrode, separate from the said gun.

7. An electron gun according to claim 4 comprising a channel in the said magnetic shielding means for passage of a connecting lead to the said electrostatic throat piece.

8. An electron gun structure comprising a magnet pole piece having a longitudinally extending hollow therein and an apertured member at one end thereof, a cathode and a focussing electrode mounted within the hollow in said pole piece and an apertured anode of non-magnetic material secured within and extending at least partially through'the aperture of the said member, the aperture of said anode being smaller than the pole piece aperture.

9. In an electron discharge device electrode means for interchanging radio frequency energy with the kinetic energy of electrons in an electron beam, said electrode means forming a tunnel through which the said beam is projected, magnetic focusing means to provide a beam confining magnetic field throughout the length of the said I tunnel and coaxial therewith, a magnetic pole piece forming part of said magnetic focusing means and having within said pole piece a longitudinally extending hollow with a closure member having a central aperture at one end thereof, an electron gun structure mounted within said hollow, said structure comprising an electron beam source emitting a beam of materially greater cross section than that of the said aperture, and electrostatic focusing means, including an anode of non-magnetic material secured and extending within the aperture of the said closure member and having an anode aperture smaller than said first mentioned aperture, the said focusing means focusing the said beam of electrons to parallelism with the said tunnel at a short distance beyond the exit of the said anode aperture in the direction towards the tunnel.

10. In an electron discharge device according to claim 9, the structure further comprising means for causing said electrostatic focusing means to focus the beam of electrons to parallelism with the said tunnel at a distance beyond the exit of the said anode aperture between one tenth and twice the radius of the said anode aperture.

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